Utilization of Stone Wool Kiln Ash in Cement-Based Materials

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Citation: Aydın, T.; Bundur, Z.B.;
Aksoy, K.; Karabıyık, B.; Perin, E.;
˙
Ince, T.; Sarı, M. Utilization of Stone
Wool Kiln Ash in Cement-Based
Materials. Mater. Proc. 2023,15, 89.
https://doi.org/10.3390/
materproc2023015089
Academic Editors: Antonios Peppas,
Christos Roumpos, Charalampos
Vasilatos and Anthimos Xenidis
Published: 5 August 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
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4.0/).
Proceeding Paper
Utilization of Stone Wool Kiln Ash in Cement-Based Materials
†
Tolga Aydın 1, Zeynep Ba¸saran Bundur 1, *, Kaan Aksoy 2, Barı¸s Karabıyık 2, Ezgi Perin 2, Türker ˙
Ince 2
and Mihriban Sarı 2
1Civil Engineering Department, Ozyegin University, 34794 Istanbul, Turkey; tolga.aydin@ozu.edu.tr
2Betek Boya ve Kimya Sanayi A.¸S., 34852 Istanbul, Turkey; kaan.aksoy@betek.com.tr (K.A.);
baris.karabiyik@betek.com.tr (B.K.); ezgi.perin@betek.com.tr (E.P.); turker.ince@betek.com.tr (T.˙
I.);
mihriban.sari@betek.com.tr (M.S.)
*Correspondence: zeynep.basaran@ozyegin.edu.tr
†Presented at the 2nd International Conference on Raw Materials and Circular Economy “RawMat2023”,
Athens, Greece, 28 August–2 September 2023.
Abstract: The main goal of this study was to validate a circular production route utilizing the waste
ash generated during stone wool production in cementitious binders. To achieve this goal, two types
of waste ash with different compositions and particle sizes were used. The performance evaluation
results showed that silica-based ashes exhibit pozzolanic behavior, reduce the amount of portlandite,
and can improve the strength of the mortar even at early ages. The ashes increased the initial setting
time regardless of their composition. The outcomes of this study create an economic value for large
volumes of material of previously zero value.
Keywords: stone wool ash; circular economy; cement-based mortar; performance
1. Introduction
For the last decade, there has been increasing awareness of the urgent need to find
solutions to initiate climate mitigation action leading to a more sustainable future. While the
cement and concrete industry plays a vital role in the European economy, their production
process significantly contributes to global warming and is responsible for 8% of CO
2
emissions [
1
,
2
]. While the chemical capture of CO
2
seems to be the only feasible approach,
high-volume cement production creates technical and economic challenges. At this time, the
pressure to reduce CO
2
emissions has only succeeded in reducing the demand for cement.
Our previous efforts in revalorizing the End-of-Life (EOL) of cementitious systems
enabled us to develop a comprehensive understanding of the activation processes in EOL
and utilize them as Supplementary Cementitious Materials (SCMs), substantially lowering
the environmental impact of the construction industry [
1
,
2
]. However, the feasibility of
regenerated EOL in cementitious systems is still open to discussion due to its effect on
strength and durability. Another possible use of EOL is geopolymer binder systems, where
reactive EOL, such as fly ash and slag, is activated through chemical synthesis [
3
]. While
there is extensive knowledge on utilizing EOL in geopolymers, it is still a major challenge
to regenerate the waste materials in cementitious systems.
Fulfilling the ambition of maintaining global carbon neutrality, utilizing alternative
zero-carbon resources is becoming more essential in the design of building materials.
Efforts to develop alternative supplementary cementitious materials are becoming popular
due to their low environmental impact and ability to reduce CO
2
emissions [
4
–
6
]. The
mineral wool waste collected from construction debris could be valorized as a potential
pozzolanic material for Portland cement concrete. However, completely circular production
can only be obtained through a “zero-waste” production route. The ashes collected from
the cokefired hot blast kiln are a significant portion of the waste stream generated from
mineral wool production. To achieve a sustainable circular production for mineral wools,
Mater. Proc. 2023,15, 89. https://doi.org/10.3390/materproc2023015089 https://www.mdpi.com/journal/materproc

Mater. Proc. 2023,15, 89 2 of 6
valorizing the waste stream obtained during production is becoming more essential for
the producer.
The main goal of this study is to validate a circular production route utilizing the
waste ash generated during mineral (stone) wool production in cementitious binders.
The performance evaluation was performed on cement-based samples where 20% of the
cement was replaced with waste ash. The results showed that W20 exhibits a pozzolanic
behavior and can improve the strength of the mortar even at early ages, while W10 does
not contribute to strength and acts as a filler material. Both ashes increased the initial
setting time and reduced the workability of the mortar. The outcome of this study creates
an economic value for large volumes of material of previously zero value while proposing
a new cementitious composite product for mineral wool producers.
2. Materials and Methods
2.1. Material Selection and Characterization
The samples were prepared with Ordinary Portland Cement (OPC) CEM I 42.5 R, and
two types of waste ash with different compositions and particle sizes were used to achieve
this goal. The ashes were classified as W10, calcite-based ash with an average particle
size of 75 microns, and W20, silicate-based ash with an average particle size of 22 microns.
Table 1summarizes the chemical composition of the binders used in the study.
Table 1. Composition of the binders, % weight in the composition.
Binder SiO2Al2O3CaO CaCO3Fe2O3MgO SO3K2O
Cement 10.70 2.65 69.90 5.00 2.35 0.71 3.37 0.70
W10 23.60 6.86 - 48.00 4.13 4.00 5.15 2.65
W20 46.40 1.96 - 8.54 2.82 7.53 4.26 13.60
Standard sand, according to the EN 196-1 norm, was used in the mortar mixes. A
polycarboxylate ether (PCE) superplasticizer (SP) was used when it became necessary
to maintain the same workability of all samples. Table 2summarizes the weight of the
ingredients in the 3 different mixes.
Table 2. Material proportions of blended cement-based mortars. W10 and W 20: Stone wool kiln ash.
Cement (g) W10 (g) W20 (g) Water (g) Standard
Sand (g)
Mix#1 600 - - 300 1800
Mix#2 480 - 120 300 1800
Mix#3 180 120 - 300 1800
2.2. Compressive Strength
The mortar samples were prepared according to ASTM C305-14 norms [
7
]. The water-
to-cement ratio (w/c) and the sand-to-cement ratio were kept at 0.5 and 3, respectively.
To calculate the strength activity index based on ASTM C311 [
8
], 20% of the binder was
replaced by kiln ashes (Table 2). The mortar samples were cast in 50
×
50
×
50 mm
cm cubes and kept in a humid environment at 21
◦
C for 24 h. Upon removal of the
molds, the samples were cured in a moist environment at ambient conditions until testing.
Compressive strength testing was conducted according to the ASTM C109-13e1 standard
at 3, 7, and 28 days on triplicate samples [9].
2.3. Vicat Needle Test
A modified Vicat Needle test was conducted to determine the setting time of the
cement pastes. The cement paste samples were prepared using a modified ASTM C191-13

Mater. Proc. 2023,15, 89 3 of 6
Standard [
10
]. Instead of determining the w/c ratio that would yield a “normal consistency”
paste, as suggested in the standard, the w/c ratio was kept constant at 0.50 to be consistent
with the compressive strength test. The initial setting of the cement paste samples was de-
termined according to the penetration depth of the Vicat needle. Analyses were conducted
based on triplicate samples.
2.4. Thermogravimetric Analysis (TGA) of Cement Pastes
TGA was performed with cement paste samples prepared with a w/c of 0.50. Follow-
ing the mixing, samples were cast into prismatic molds. The specimens were initially cured
at 100% relative humidity at 21
◦
C for 24 h. Upon removal of the molds, the samples were
cured in a moist environment at ambient conditions until testing. TGA testing was con-
ducted on the samples at 7 and 28 days. The samples were tested with TGA-DTA Analyzer
(Perkin Elmer, Shelton, CT, USA). The analysis was conducted by heating the samples from
40
◦
C to 1100
◦
C, and mass loss was recorded as a function of time. The decomposition
of calcium hydroxide (portlandite) was determined between 450 and 550
◦
C, and calcium
carbonate (CaCO3) decarbonization was measured between 700 ◦C and 900 ◦C [11].
3. Results and Discussion
3.1. Pozzolanicity of Stone Wool Kiln Ash and Its Impact on the Compressive Strength
Figure 1displays the results of the compressive strength and strength activity index
(SAI) tests. Based on the findings, W20 possesses pozzolanic activity, while W10 does
not contribute to compressive strength development. The pozzolanic activity of the stone
wool kiln ash was assessed based on the ASTM C 311 [
8
] and C 618 [
12
] standards. ASTM
C618 specifies that the activity of mortars containing pozzolans at 7 and 28 days must not
fall below 75%, represented by the line labeled as minimum. The evaluation of SAI also
revealed that W20 exhibits pozzolanic activity while W10 does not. This was attributed to
the different chemical and physical properties of these two different ashes.
W20 is silicate-based ash with an average particle size of 22 microns. Even though
W10 has 24% SiO
2
in its composition, the coarser particle size decreases its reactivity. W10
acts as a filler instead of a binder. At last, the influence of W20 was more pronounced after
7 days of mixing.
3.2. Impact on Fresh State Properties: Initial Setting Time
Figure 2represents the modified Vicat needle test results. The different initial setting
times were evaluated by comparing them with the reference Mix #1 samples with an initial
setting time of approximately 6 h. Incorporating both stone wool kiln ashes significantly
increased the initial setting time. Replacing 20% of the binder with W10 and W20 resulted
in approximately 10.5 h of setting time. This indicates that neither of these ashes possesses
hydraulic activity. Based on the data obtained from the strength and setting time tests, it is
evident that W20 is a silicate-based pozzolan resembling the behavior of F-type fly ash and
that W10 is a calcite-based filler like limestone. Further, mix design optimization should be
performed considering the characteristics of these waste materials.
3.3. Impact on Composition: Calcium Carbonate and Calcium Hydroxide Content
Table 3summarizes the mass percentages of the portlandite and CaCO
3
calculated
via TGA. Based on the data obtained, it is evident that the incorporation of W20 in the
mix decreased the amount of portlandite even at seven days. The results align with the
compressive strength results, showing that W20 acts as a pozzolanic material, reacts with
portlandite even as early as seven days after mixing, and starts improving strength.

Mater. Proc. 2023,15, 89 4 of 6
Mater. Proc. 2023, 15, 89 4 of 6
Figure 1. Influence of stone wool kiln ash on compressive strength. (a) Average compressive
strength at 3, 7, and 28 days. (b) Strength Activity Index.
W20 is silicate-based ash with an average particle size of 22 microns. Even though
W10 has 24% SiO
2
in its composition, the coarser particle size decreases its reactivity. W10
acts as a filler instead of a binder. At last, the influence of W20 was more pronounced after
7 days of mixing.
3.2. Impact on Fresh State Properties: Initial Seing Time
Figure 2 represents the modified Vicat needle test results. The different initial seing
times were evaluated by comparing them with the reference Mix #1 samples with an initial
seing time of approximately 6 hours. Incorporating both stone wool kiln ashes signifi-
cantly increased the initial seing time. Replacing 20% of the binder with W10 and W20
Figure 1. Influence of stone wool kiln ash on compressive strength. (a) Average compressive strength
at 3, 7, and 28 days. (b) Strength Activity Index.
Table 3. Portlandite and CaCO3content in cement paste samples.
7-Days 28-Days
Portlandite (%) CaCO3(%) Portlandite (%) CaCO3(%)
Mix#1 6.5 2.3 7.0 2.7
Mix#2 5.3 3.2 5.0 2.7
Mix#3 6.1 4.1 5.3 4.1

Mater. Proc. 2023,15, 89 5 of 6
Mater. Proc. 2023, 15, 89 5 of 6
resulted in approximately 10.5 hours of seing time. This indicates that neither of these
ashes possesses hydraulic activity. Based on the data obtained from the strength and set-
ting time tests, it is evident that W20 is a silicate-based pozzolan resembling the behavior
of F-type fly ash and that W10 is a calcite-based filler like limestone. Further, mix design
optimization should be performed considering the characteristics of these waste materials.
Figure 2. Modified ASTM C191-13 initial seing times of different cement paste mixes. The bars
represent the average initial seing time values based on triplicate cement paste samples, and the
error bars represent the one standard deviation.
3.3. Impact on Composition: Calcium Carbonate and Calcium Hydroxide Content
Table 3 summarizes the mass percentages of the portlandite and CaCO
3
calculated
via TGA. Based on the data obtained, it is evident that the incorporation of W20 in the mix
decreased the amount of portlandite even at seven days. The results align with the com-
pressive strength results, showing that W20 acts as a pozzolanic material, reacts with port-
landite even as early as seven days after mixing, and starts improving strength.
Further evaluation has to be performed to understand the materialʹs performance at
later ages (i.e., one year). Similarly, Mix #3, with a 20% W10 replacement in the binder,
resulted in a CaCO
3
content due to the composition of W10. While W10 is unsuitable for
use as a binder replacement, it is still possible to utilize the waste material as a filler in
cement-based materials.
Table 3. Portlandite and CaCO
3
content in cement paste samples.
7-Days 28-Days
Portlandite (%) CaCO
3
(%) Portlandite (%) CaCO
3
(%)
Mix#1 6.5 2.3 7.0 2.7
Mix#2 5.3 3.2 5.0 2.7
Mix#3 6.1 4.1 5.3 4.1
4. Conclusions
This study was undertaken to revalorize stone wool kiln ash as a pozzolan in cement-
based materials. To achieve this goal, two types of waste ash with different compositions
and particle sizes were used. The ashes were classified as W10, calcite-based ash with an
Figure 2. Modified ASTM C191-13 initial setting times of different cement paste mixes. The bars
represent the average initial setting time values based on triplicate cement paste samples, and the
error bars represent the one standard deviation.
Further evaluation has to be performed to understand the material’s performance at
later ages (i.e., one year). Similarly, Mix #3, with a 20% W10 replacement in the binder,
resulted in a CaCO
3
content due to the composition of W10. While W10 is unsuitable for
use as a binder replacement, it is still possible to utilize the waste material as a filler in
cement-based materials.
4. Conclusions
This study was undertaken to revalorize stone wool kiln ash as a pozzolan in cement-
based materials. To achieve this goal, two types of waste ash with different compositions
and particle sizes were used. The ashes were classified as W10, calcite-based ash with an
average particle size of 75 microns, and W20, silicate-based ash with an average particle
size of 22 microns. The results showed that W20, with its higher silicate content and finer
particle size, could exhibit pozzolanic behavior and react with portlandite. The pozzolanic
tendency of W20 resulted in an increase in compressive strength. In contrast, W10, with its
coarser particle size and low alumina silicate content, possessed no reactivity and acted
as a filler in the system. Both ashes resulted in a longer setting time compared to the
control sample. The outcomes will contribute to the circular economy by proposing a
circular production method for the stone wool industry to utilize their waste stream and
RM to design building elements for practical applications. The outcomes of this project will
provide alternative solutions to climate change and the efficient use of local raw materials
for alternative building materials.
Author Contributions: T.A.: Conceptualization of the work, Experimental analysis, Data curation,
and analysis, Methodology, Drafting the plots and tables, and Writing—original draft; Z.B.B.: Concep-
tualization of the work, Funding acquisition, Project management, and administration, Methodology,
Supervision and Writing—Review and Editing; K.A., B.K., E.P., T.˙
I. and M.S.: All authors have read
and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.

Mater. Proc. 2023,15, 89 6 of 6
Data Availability Statement: The authors confirm that the data supporting the findings of this study
are available within the article.
Conflicts of Interest: Authors K.A., B.K., E.P., T.˙
I. and M.S. are employed in Betek Boya ve Kimya
Sanayi A.¸S.
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